[Release 8] update release table (#4551)

README.md

 
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 7`. Click
+**Our latest, stable release is `Release 8`. Click
 [here](https://github.com/Unity-Technologies/ml-agents/tree/release_8_docs/docs/Readme.md)
 to get started with the latest release of ML-Agents.**
 
 | **Version** | **Release Date** | **Source** | **Documentation** | **Download** |
 |:-------:|:------:|:-------------:|:-------:|:------------:|
 | **master (unstable)** | -- | [source](https://github.com/Unity-Technologies/ml-agents/tree/master) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/master/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/master.zip) |
-| **Release 7** | **September 16, 2020** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_7)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_7_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_7.zip)** |
+| **Release 8** | **October 14, 2020** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_8)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_8_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_8.zip)** |
+| **Release 7** | September 16, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_7) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_7_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_7.zip) |
-| **Release 1** | April 30, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_1) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_1_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_1.zip) |
 
 ## Citation
 

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
-
-### Major Changes
-#### com.unity.ml-agents (C#)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
-### Minor Changes
-#### com.unity.ml-agents (C#)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
-### Bug Fixes
-#### com.unity.ml-agents (C#)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
 
 ## [1.5.0-preview] - 2020-10-14
 ### Major Changes

utils/make_readme_table.py

     ReleaseInfo("release_5", "1.2.1", "0.18.1", "July 31, 2020"),
     ReleaseInfo("release_6", "1.3.0", "0.19.0", "August 12, 2020"),
     ReleaseInfo("release_7", "1.4.0", "0.20.0", "September 16, 2020"),
+    ReleaseInfo("release_8", "1.5.0", "0.21.0", "October 14, 2020"),
 ]
 
 MAX_DAYS = 150  # do not print releases older than this many days